SU‐GG‐I‐27: Image Artifacts Caused by the Extra‐Focal Spot of An X‐Ray Tube in Cone‐Beam Computed Tomography

Purpose: We investigate the reconstruction artifacts induced by extra‐focal spot of x‐ray tube in cone‐beam computed tomography (CBCT). Method and Materials: The extra‐focal radiation problem of an x‐ray tube has been investigated for more than thirty years. However, these investigations have been mostly limited to fan‐beam computed tomography (FBCT) where only the quasi‐one‐dimensional source distribution matters. In contrast, CBCT is more vulnerable to the problem of extra‐focal field than FBCT because the quasi‐two‐dimensional distribution of source is significant. Extra‐focal radiation has been attributed to two physical processes: secondary electrons and field emission. Specifically, field emission of electrons from a cold cathode may create a well‐defined extra‐focal spot that can emit a substantial amount of x‐rays. We have observed a non‐negligible amount of x‐ray flux from an off‐focal spot in the x‐ray tube of an on‐board imager used for image‐guided radiation therapy, and have performed a numerical study to simulate the image artifacts induced by this strong extra‐focal spot. The 3‐D Shepp‐Logan phantom was used as an imaging object and circular cone‐beam projections were made with two focal source points separated by 8 mm along the rotation axis direction. The off center focal spot had an intensity of 10% that of the central spot. Results: We observed image artifacts such as overlaid structure of the object components in the coronal slice images and ghost objects in some transverse slice images. Conclusion: In addition to the well‐known image artifacts, due to spread extra‐focal field of an x‐ray tube, which include blurring of the images, well‐defined extra‐focal spots can introduce additional image artifacts such as overlaying structures and ghost objects.